Frames assembled in a ladder configuration and comprising steel material are used in medium-size and large-size vehicles. Springs, axles, the engine, a plurality of transmission devices, brake devices, body components and the like are mounted on such frames. Furthermore, air tanks, fuel tanks, batteries, electrical wires, conduits, and the like are also mounted.
In general, such frames are formed by mounting a plurality of steel cross members in a ladder configuration between a pair of steel side rails disposed in parallel with each other. The steel material of each of the pair of side rails is formed so as to have a U-shaped cross section, and the pair of side rails is assembled such that the open sides of the U-shaped cross sections thereof are mutually opposed.
In conventional manufacturing sequences for such vehicles, this ladder-configuration frame is assembled first of all using a pair of side rails and a plurality of cross members. Next, the assembled frame is introduced into a manufacturing line, and a variety of devices and components are mounted thereon. In well known and commonly used manufacturing-line formats, such frames are temporarily held on the manufacturing line in a vertically inverted condition, or in other words, upside-down. In this condition, front and rear springs, axles, and other suspension devices and components that will be disposed in the lower part of the completed vehicle are mounted first of all to form a chassis module. Next, this chassis module, to which a large number of heavy components have been assembled, is lifted in its entirety by a large crane, inverted vertically through rotation thereof by 180 degrees about an axis parallel with a longitudinal direction thereof, and returned to the manufacturing line by the crane. All subsequent steps are performed in a normal orientation without further inversion.
On the subsequent manufacturing line, springs, axles, air tanks, the engine, and transmission devices, etc. are mounted, and the cab and fuel tank, etc. are then mounted. In this type of work step, assisting devices such as cranes disposed along the processing line are used with the engine, transmission devices, and other heavy components, and the components and devices are disposed in the vicinity of the frame's specific mounting positions. The mounting of components such as tanks, conduit tubes, and wire harnesses used for electrical wiring is carried out manually by workers who remove each component or device from a provided component box, carry the component to the designated mounting position thereof, and while holding the component or device in one hand or adopting a similar posture, perform the necessary operations for mounting thereof on the frame.
This type of configuration of a vehicle manufacturing step constitutes a widely known technique and is, for example, described in detail in non-patent document 1 as defined herein below.    Non-patent document 1: Review of Automotive Engineering (Jidosha Gijutsu Handobukku) No. 4; Manufacturing, Quality, and Maintenance Edition, edited and released by Society of Automotive Engineers of Japan, Inc. released Sep. 1, 1991. In particular, see content of page 284 and thereabouts.